Cathode sputtering is widely used for depositing thin films of material onto substrates. The process involves vaporizing a material by ion bombardment of a target which forms part of a cathode assembly in an evacuated chamber containing an inert gas such as argon. A high voltage electric field is applied between the cathode assembly and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode. The positive gas ions are attracted to the cathode surface, and atoms of material dislodged when the ions strike the target then traverse the enclosure and deposit as a thin film onto a substrate positioned on a support maintained at or near anode potential.
Although the sputtering process can be carried out solely in an electric field, substantially increased deposition rates are possible with magnetron sputtering in which an arched magnetic field, formed in a closed loop over the surface of the sputtering target, is superimposed on the electric field. The arched closed-loop magnetic field traps electrons in an annular region adjacent to the surface of the target, thereby multiplying the collisions between electrons and gas atoms to produce a corresponding increase in the number of ions in that region.
In the conventional planar target cathode assembly, a flat target plate composed of the material which is to be deposited onto the substrates is clamped in place by nonmagnetic clamping rings positioned radially inside and outside of the target. A magnetic backing plate supports the clamping rings and target and is magnetically coupled to a source of a magnetic field, such as electromagnets or permanent magnets. A magnetic field is produced that arches in the form of a tunnel or closed loop extending along the exposed face of the target. Such a target assembly is shown by Boys in U.S. Pat. No. 4,761,218 entitled "Sputter Coating Source Having Plural Target Rings". With this design, the target is supported on magnet pole lips that extend under inner and outer edges of the base surface of the target.
A drawback of the conventional flat plate target is that erosion occurs in a relatively narrow ring-shaped region corresponding to the shape of the closed-loop magnetic field. The reason for this is that since the path of an electron leaving the target is approximately perpendicular to the surface, it is only the component of the arched magnetic field that is parallel to the surface that produces any deflection of the electron path along the magnetic "tunnel". Over the magnetic poles this parallel component becomes vanishingly small, thereby allowing the electrons to escape from the field. Thus, ionizing efficiency over the poles is very low, and sputtering rates there are correspondingly small. As a result, only the portion of the total target material in the so-called "race track" region is consumed before the target must be replaced. Class et al., in U.S. Pat. No. 4,198,283 entitled "Magnetron Sputtering Target and Cathode Assembly", describes a target assembly utilizing such a conventional type of target.
Another drawback of the conventional annular target, particularly a circular target, is that the flux density near the exposed surface of the target deceases with distance from the center axis of the circular or annular target. Thus, the flux concentration is greater near the inner edge of the target and less near the outer edge. This also contributes to reduced erosion at the outer periphery of the target.
In order to increase the percentage of usable material of a target, the shape of the target has been redesigned to correspond to the actual erosion characteristics during sputtering. Rainey, in U.S. Pat. No. 4,100,055 entitled "Target Profile for Sputtering Apparatus", discloses a target that has increasing thickness with distance from the target center. This accommodates erosion that is greater in the outer perimeter region. The target assemblies show magnetic fields that extend from a pole member adjacent the elevated outer surface of the target to a pole plate supporting the target. This pole arrangement results in magnetic field lines that are parallel to the target surface at only the outer periphery.
Pierce et al., in U.S. Pat. No. 4,385,979 entitled "Target Assemblies of Special Materials for Use in Sputter Coating Apparatus", discloses an improvement on the target assembly design of Rainey. A pole piece is positioned adjacent the inside surface of the target, rather than under the target, producing flux lines that extend further along the target sputtering surface. This results in erosion of the target closer to its center, which is the thickest part of the target.
These prior art approaches have thus been directed to controlling the shape of the target to maximize the yield from the target. There thus remains a need for a target assembly that has a magnetic field that is distributed selectively across the target sputtering surface to produce a desired erosion profile.
There is also a need for a target assembly having a magnetic field that extends generally parallel to the sputtering surface of a target across more of its width in order to distribute the erosion across a greater proportion of the surface.